A wire electrode is normally supplied to a workpiece from a wire bobbin, running along a fixed wire conveying path including a tension applying device, to take-up rollers from which used wire electrode is discharged to an appropriate bucket. If a voltage pulse is applied between the wire electrode and the workpiece, electric discharge is caused between the wire electrode, as it is held in tension while traveling between a pair of wire guides and the workpiece. A wire electric discharge machine causes the wire electrode and the workpiece to move relative to each other within a horizontal X-Y plane in order to form the workpiece into a desired profile. The speed of travel of the wire electrode is controlled by a motor causing rotation of the take-up rollers, and tension on the wire electrode is provided by the tension applying device. In many instances, a wire electrode having a diameter of about 0.20 to 0.35 mm is used, but sometimes, a fine wire, having a diameter of about 0.15 to 0.05 mm, is used. The tension applying device adjusts the tension according to the diameter of the wire electrode. For example, a tension of about 1200 g may be applied to a wire electrode having a diameter of 0.20 mm, and a tension of about 50 g may be applied to a wire electrode having a diameter of 0.03 mm. In order to improve surface roughness and dimensional accuracy, a tension applying device has been proposed in order to accurately apply the appropriate tension according to the diameter to the wire electrode, and to prevent variations in tension.
Japanese Patent 8-336 discloses a tension applying device provided with a brake roller 47 linked to a high torque electromagnetic brake 3 via a clutch 7, and a pinch roller 46 which contact(s) a peripheral surface of the brake roller 47 and which is linked to a low torque electromagnetic brake 1, as shown in FIG. 10. A wire electrode 6 is wound in an S-shape around the rollers 46 and 47. When a relatively thick wire electrode 6 is used, the relatively large braking torque of the high torque electromagnetic brake 3 is transmitted to the brake roller 47. On the other hand, when a fine wire electrode 6 is used, the clutch 7 is disengaged and only the relatively small braking torque of the low torque electromagnetic brake 1 is transmitted to the pinch roller 46.
As shown in FIG. 11, Japanese Patent No. 2686783 discloses a tension applying device having a switch 8 for connecting a brake roller 47 to a large brake 3 when a relatively thick wire electrode 6 is used, and for connecting the brake roller 47 a small brake 1 when a thin wire electrode 6 is used. A similar device is also disclosed in U.S. Pat. No. 5,422,454.